System and method for tissue characterization using fast quantitative spin-lock magnetization transfer imaging

A method for quantifying magnetization transfer using a magnetic resonance imaging (MRI) apparatus, the method comprising:

• performing a first image acquisition process to produce a first MR image, wherein the first image acquisition process includes applying a first off-resonance spin-lock pulse having a first RF amplitude (ω1(1)), a first frequency offset (Δω(1)), and a first time of spin-lock (TSL(1)), wherein the first image acquisition process is used to determine a first measured magnetization M(1) that depends on a first relaxation rate in the rotating frame R1ρ(1);
• performing a second image acquisition process to produce a second MR image, wherein the second image acquisition process includes applying a second off-resonance spin-lock pulse having a second RF amplitude (ω1(2)), a second frequency offset (Δω(2)), and a second time of spin-lock (TSL(2)), wherein the second image acquisition process is used to determine a second measured magnetization M(2) that depends on a second relaxation rate in the rotating frame R1ρ(2) and wherein one or both of the following conditions is satisfied:
• (a) R1ρ(ω1, Δω)TSL<<1 for each of the first spin-lock pulse and the second spin-lock pulse, wherein R1ρ(ω1, Δω) is a relaxation rate in the rotating frame for a given RF amplitude ω1 and frequency offset Δω;
• or
• (b) each of the first and second image acquisition processes includes a magnetization reset RF pulse prior to the off-resonance spin-lock pulse and a recovery time (Trec) between the magnetization reset RF pulse and the beginning of the off-resonance spin-lock pulse is greater than a longitudinal relaxation time (T1) of water protons;
• performing a third image acquisition process to produce a third MR image, wherein the third image acquisition process includes applying a third off-resonance spin-lock pulse having a third RF amplitude (ω1(3)), a third frequency offset (Δω(3)), and a third time of spin-lock (TSL(3)) that is equal to zero, wherein the third image acquisition process is used to determine a third measured magnetization Mini that represents the initial magnetization just before spin-lock;
• and
• computing, based on the first, second, and third MR images, one or more parameters of magnetization transfer, wherein the one or more parameters of magnetization transfer include a parameter Rmts1 defined as Rmts1=R1ρ(2)−R1ρ(1), wherein computing the parameter Rmts1 includes computing:
• R
• mtsl
• =
• 1
• TSL
• (
• 2
• )
• -
• 1
• TSL
• (
• 1
• )
• +
• M
• (
• 1
• )
• ⁢
• TSL
• (
• 2
• )
• -
• M
• (
• 2
• )
• ⁢
• TSL
• (
• 1
• )
• M
• i
• ⁢
• n
• ⁢
• i
• ⁢
• TSL
• (
• 1
• )
• ⁢
• TSL
• (
• 2
• )
• .

A magnetic resonance imaging (MRI) system comprising:

• an MRI apparatus having a magnet, a gradient coil, and one or more radiofrequency (RF) coils;
• and
• a computer communicably coupled to the MRI apparatus, the computer having a processor, a memory, and a user interface, the processor being configured to:
• perform a first image acquisition process to produce a first MR image, wherein the first image acquisition process includes applying a first off-resonance spin-lock pulse having a first RF amplitude (ω1(1)), a first frequency offset (Δω(1)), and a first time of spin-lock (TSL(1));
• perform a second image acquisition process to produce a second MR image, wherein the second image acquisition process includes applying a second off-resonance spin-lock pulse having a second RF amplitude (ω1(2)), a second frequency offset (Δω(2)), and a second time of spin-lock (TSL(2)), wherein the first time of spin-lock TSL(1) and the second time of spin-lock TSL(2) are equal to the same time of spin-lock (TSL),
• wherein the first RF amplitude ω1(1), the first frequency offset Δω(1), the second RF amplitude (ω1(2)), and the second frequency offset (Δω(2)) are chosen such that Δω(1)/ω1(1)=Δω(2)/ω1(2) and wherein one or both of the following conditions is satisfied:
• (a) R1ρ(ω1, Δω)TSL<<1 for each of the first spin-lock pulse and the second spin-lock pulse, wherein R1ρ(ω1, Δω) is a relaxation rate in the rotating frame for a given RF amplitude ω1 and frequency offset Δw;
• or
• (b) each of the first and second image acquisition processes includes a magnetization reset RF pulse prior to the off-resonance spin-lock pulse and a recovery time (Trec) between the magnetization reset RF pulse and the beginning of the off-resonance spin-lock pulse is greater than a longitudinal relaxation time (T1) of water protons;
• and
• compute, based on the first and second MR images, one or more parameters of magnetization transfer, wherein the one or more parameters of magnetization transfer include a parameter Rmts1 defined as Rmts1=R1ρ(2)−R1ρ(1), wherein:
• R1ρ(1) is a first relaxation rate in the rotating frame responsive to a spin-lock pulse having the first RF amplitude ω1(1) and the first frequency offset Δω(1);
• and
• R1ρ(2) is a second relaxation rate in the rotating frame responsive to a spin-lock pulse having the second RF amplitude ω1(2) and the second frequency offset Δω(2),
• wherein computing the parameter Rmts1 includes computing:
• R
• mtsl
• =
• -
• log
• ⁢
• M
• (
• 2
• )
• M
• (
• 1
• )
• /
• TSL
• ,
• wherein M(1) corresponds to the first image and M(2) corresponds to the second image.